1. Technical Field of this Invention
The present Invention relates to novel devices and methods to minimize the production of sand in subterranean environments. In particular, in poorly consolidated formations, for instance, sand is co-produced along with the desired fluid (e.g., oil); sand production is undesirable, hence in the present Invention, elliptically shaped perforations of a particular orientation (in preferred embodiments) are created through the casing that lines the wellbore (as well as created in an uncased formation) and that penetrate the formation rock, to improve the stability of the perforation tunnel, and therefore minimizing sand intrusion (or the intrusion of disaggregated formation particles generally, in the case of, e.g., carbonate formations).
2. Prior Art
In the production of oil and gas from a subterranean reservoir, one persistent problem in certain types of reservoirs is that sand is also produced along with the hydrocarbon. The present Invention is directed to novel techniques to control the coproduction of sand with hydrocarbons (i.e., "sand control"). Obviously, the goal in oil and gas production is to move the hydrocarbon from the underground formation where it resides, to a wellbore drilled in the earth, and eventually to the surface, for transportation and eventual refining. Many hydrocarbon-bearing formations are sandstone, and many of those are poorly consolidated sandstone, which means that the sand grains that comprise the geologic formation are loosely held together. In certain formations, sand flows from the formation along with the oil--this may occur initially, or later in the life of the well. This "sand production" is highly undesirable. For one thing, sand is a harsh abrasive and so abrades just about everything it comes in contact with--production string (generally steel tubing) lining the wellbore, aboveground pipelines, and so forth. If enough sand is co-produced with the oil then it is not even suitable for processing, or only at substantial additional expense.
Therefore, numerous techniques have evolved to deal with the problem; they are roughly divisible into two categories: mechanical and non-mechanical. The primary mechanical technique is known as "gravel packing." A particularly sophisticated type of gravel packing is AllPAC, a patented technology jointly developed by Mobil and Schlumberger and exclusively licensed to Schlumberger. (See, e.g., L. G. Jones, Alternate-Path Gravel Packing, SPE 22796 (1991)). The idea behind gravel packing is to place a permeable screen inside the wellbore between the casing (if there is one) and the wellbore, next the annulus formed by the screen and casing/wellbore is filled with gravel. (Alternatively, a screen without gravel is sometimes used; also, sometimes "pre-packed" screens are used, in which the gravel is placed in the screen before it is placed in the wellbore). The purpose of the screen is to hold the gravel in place, and the purpose of the gravel (and screen) is to remove the sand, yet allow the oil (or gas) to migrate through the gravel pack, into the wellbore and eventually to the surface.
Although gravel packing is a venerable sand control technique, still widely relied upon, it has numerous very substantial disadvantages. First, screens are very expensive; this expense is naturally exacerbated in horizontal wells, where the amount of screen needed frequently exceeds a thousand feet. Moreover, placing a screen in a horizontal section is time-consuming and expensive. Second, a rig or mast must be used to place screen in a wellbore; rig rates are quite often very high, particularly offshore (e.g., in the North Sea, they can exceed $100,000/day). Third, whatever benefit--in reduced sand production--is derived from the gravel pack, the fact remains that it is a choke on production, often substantially reducing potential production rates. Related to this, screens can become plugged--e.g., by fines (very small grain sands) may become affixed to the screen face where they form a "filter cake," which can severely inhibit, or even halt production.
The second major category of sand control techniques relates not to impeding the flow of sand via a filter (gravel pack) but instead relates to improving the near-wellbore integrity of the formation so that less sand flows into the wellbore. For the most part, these techniques involve somehow consolidating the sandstone around the wellbore--i.e., cementing the sand grains together so that they do not flow along with the oil, into the wellbore. To do this requires some sort of cementing material, such as a furan resin or epoxy resin. For instance, U.S. Pat. No. 5,551,514, assigned to Schlumberger, discloses and claims, e.g., a method of controlling sand production by consolidating the near-wellbore formation by injecting a resin into that region of the formation. Next, that portion of the formation is hydraulically fractured--i.e., sufficient fluid is pumped into the formation to cause it to split. The idea is that formation consolidation is achieved (via the resin) but not at the expense of reduced hydrocarbon production (since the formation is actually stimulated by the fracture).
These non-mechanical (or chemical) sand control techniques suffer predictably, from reduced permeability in the region of the formation where the consolidation is placed. In other words, while the idea behind these types of treatments is to cement the contiguous sand grains together, but not leave the resin in the pore spaces (where the oil must flow), most treatments rarely approach this ideal. Indeed, to remove the resin from the pore spaces requires that still more chemicals be pumped into the reservoir to "flush" the resin from the pore spaces; still more chemicals are required in some cases, to "pre-treat" the sand grains so that the resin sticks to the sand grains preferentially (hence resists the flushing step) but is readily removable from the (un-pre-treated) pore spaces.
The present Invention is also directed to sand control, but fits in neither of these categories. That is, it is neither mechanical nor chemical. The present Invention shall be explained below with reference to certain prior art.
One of the first steps in oil and gas production is drilling a wellbore into the hydrocarbon-bearing formation. Next, a casing (liner), generally steel, is inserted into the wellbore, and forms a gap between the casing and wellbore, typically referred to as the annulus. Once the casing is inserted into the wellbore, it is then cemented in place, by pumping cement into the annulus. The reasons for doing this are many, but essentially, a liner helps ensure the integrity of the wellbore, i.e., so that it does not collapse; another reason for the wellbore liner is to isolate different geologic zones, e.g., an oil-bearing zone from an (undesirable water-bearing zone). By placing a liner in the wellbore and cementing the liner to the wellbore, then selectively placing holes in a liner cemented to the wellbore, one can effectively isolate certain portions of the subsurface, for instance to avoid the co-production of water along with oil.
That process of selectively placing holes in the liner and cement so that oil and gas can flow from the formation into the wellbore and eventually to the surface is generally known as "perforating." One common way to do this is to lower a perforating gun into the wellbore using a wireline or slickline, to the desired depth, then detonate a shaped charge within the gun. The shaped charge creates a hole in the adjacent wellbore liner and formation behind the liner. This hole is known as a "perforation." Perforating guns are comprised of a shaped charge mounted on a base. U.S. Pat. No. 5,816,343, assigned to Schlumberger Technology Corporation, incorporated by reference in its entirety, discusses prior art perforating systems (e.g., col. 1., 1. 17).
We are aware of one group that has examined the role of perforation stability on sand production. See, N. Morita, Fracturing, Frac Packing, and Formation Failure Control: Can Screenless Completions Prevent Sand Production? SPE 36457 (1998). For instance, these investigators note that "Perforation stability significantly improves if the perforations are shot in the maximum horizontal in-situ stress direction, if the two principal horizontal stresses are significantly different, or the perforations can be shot in the well azimuth direction if the well is highly inclined." Id. at 395. Yet this articles neither discloses nor suggests a particular perforation geometry (other than circular) and particular orientation (since that only has meaning if the perforations are non-circular)
In addition, U.S. Pat. No. 5,386,875, Method for Controlling Sand Production of Relatively Unconsolidated Formations (assigned to Halliburton) is directed to a method for controlling sand production by optimizing perforation orientation. This patent differs from the present Invention in part because the '875 patent neither claims, discloses, nor suggests optimizing the geometry of the perforations (i.e., their shape), but instead is directed solely to their orientation around the well casing.
The present Invention relates to a method of controlling the production of sand, based on optimizing the geometry and the orientation of perforations. Hence, this method suffers from none of the difficulties which plague conventional sand control techniques--e.g., cost (screens) and diminished permeability (resin consolidation).